[mlir][irdl] Add `irdl.c_pred`

`irdl.c_pred` is an attribute constraint defined by a C++ predicate.
Contrary to the other constraints, this operation cannot be used in
dialects that are registered at runtime. Its principal use is to
share dialect definitions that are defined in C++ or ODS.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D152301